Formation of metal nanowires for use as variable-range hydrogen sensors

ABSTRACT

The present invention provides for variable-range hydrogen sensors and methods for making same. Such variable-range hydrogen sensors comprise a series of fabricated Pd—Ag (palladium-silver) nanowires—each wire of the series having a different Ag to Pd ratio—with nanobreakjunctions in them and wherein the nanowires have predefined dimensions and orientation. When the nanowires are exposed to H 2 , their lattice swells when the H 2  concentration reaches a threshold value (unique to that particular ratio of Pd to Ag). This causes the nanobreakjunctions to close leading to a 6-8 orders of magnitude decrease in the resistance along the length of the wire and providing a sensing mechanism for a range of hydrogen concentrations.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 10/909,797, which is assigned to the assignee ofthe present invention, which was filed on Jul. 30, 2004 now U.S. Pat.No. 7,104,111, which is a divisional application of U.S. patentapplication Ser. No. 10/651,220, which was filed on Aug. 28, 2003 nowU.S. Pat. No. 6,849,911, which claims priority under 35 U.S.C. §119(e)to the following U.S. Patent Application No. 60/407,141, which was filedon Aug. 30, 2002.

TECHNICAL FIELD

The present invention relates in general to the fabrication of metalnanowires, and in particular to hydrogen gas sensors comprising suchnanowires.

BACKGROUND INFORMATION

Hydrogen gas (H₂) is widely used in industrial and laboratory settings.Because of its high-flammability in air, the need to detect hydrogen gasat levels below its lower explosive limit (LEL is 4% at 25° C.) is ofconsiderable importance. The use of palladium (Pd) as a hydrogen sensoris well-established (F. A. Lewis, “The Palladium Hydrogen System,”Academic Press, New York, 1967.) and based on the increased resistancerealized when hydrogen dissolves into the metal creating a palladiumhydride which has a lower degree of conductivity than pure palladium.

Recently, a palladium nanowire (also known as a “mesowire,” wheremesoscopic structures are characterized by a length scale ranging fromthe tens of nanometers to micrometers) sensor has been described whichoperates with an inverse response, i.e., it realizes a decreasedresistance when exposed to hydrogen (F. Favier, E. C. Walter, M. P.Zach, T. Benter, R. M. Penner “Hydrogen Sensors and Switches fromElectrodeposited Palladium Mesowire Arrays,” Science, 293, p. 2227-2231,2001). Such nanowires are electrodeposited, from solutions of palladiumchloride (PdCl₂) and perchloric acid (HClO₄), onto anelectrically-biased graphite step ledge (presumably, these terraced stepledges produce an enhanced field leading to selective deposition). Onceformed, these nanowires are transferred to an insulating glass substrateusing a cyanoacrylate film. The diameters of these wires are reportedlyas small as 55 nanometers (nm) and they possess gaps or break-junctionswhich impart them with high resistance. When hydrogen is introduced, apalladium-hydride (PdH_(x)) forms. At room temperature (25° C.), thereis a crystalline phase change from α to β when the concentration ofhydrogen in air reaches 2% (15.2 Torr). Associated with this phasechange is a corresponding 3-5% increase in the lattice parameter of themetal which leads to a “swelling” of the nanowire, thus bridging thenanogap breakjunctions (nanobreakjunctions) and leading to an overalldecrease in the resistance along the length of the nanowire. Theresistance change that occurs is between 6 and 8 orders of magnitude(typical devices see 1×10⁻¹¹ amps in the “off” state, and 1×10⁻⁴ amps inthe “on” state). This behavior is unique to nanowires possessing suchnanogap breakjunctions. Fortunately, for sensor applications, these gapsre-open when the nanowires are removed from the hydrogen-containingenvironment, and the swollen nanowires revert back to their pre-swollenstate.

The above-mentioned nanowire sensors have three primary deficiencieswhich can be improved upon. The first deficiency is the reliance onterraced graphite step ledges to form the nanowires. This limits theability to pattern the nanowires into an arrangement of one's ownchoosing, i.e., it limits the length and orientation of the nanowires.The second deficiency lies with the need to transfer the nanowires fromthe conducting graphite surface to an insulating glass substrate using acyanoacrylate “glue.” Such transfer steps could damage the nanowires.Lastly, there are hydrogen concentration and temperature constraintswhich present, perhaps, the greatest deficiency in the current state ofthe art. At 25° C., for example, there is no H₂ concentration range overwhich this sensor can detect-merely a 2% threshold. By 50° C., thisthreshold moves up to 4-5% H₂ in air, which is above the lower explosivelimit. Consequently, such nanowire sensors essentially provide only ahydrogen detection capability within a very narrow temperature range.

As a result of the foregoing, there is a need for a method that permitsthe ordered patterning of nanowires on a surface in a predefined way,and for a method that eliminates the need for the nanowires, onceformed, to be transferred to another substrate. There is also a need fora method that overcomes the temperature/threshold concentrationlimitations of current hydrogen sensors and allows for a range of H₂concentrations to be determined at any given temperature, and whichallows for a wider range of operating temperatures such that the sensoris capable of detecting H₂ below its lower explosive limit. Such adevice could serve as a variable or dynamic-range hydrogen sensor.

SUMMARY OF THE INVENTION

The present invention provides for variable-range hydrogen sensors andmethods for making same. Such variable-range hydrogen sensors compriseone or more fabricated Pd—Ag (palladium-silver) nanowires—each wirehaving the same or a different Ag to Pd ratio—with nanobreakjunctions inthem and wherein the nanowires have predefined dimensions andorientation. When the nanowires are exposed to H₂, their lattice swellswhen the H₂ concentration reaches a threshold value (unique to thatparticular ratio of Pd to Ag). This causes the nanobreakjunctions toclose leading to a 6-8 orders of magnitude decrease in the resistancealong the length of the wire and providing a sensing mechanism for arange of hydrogen concentrations.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates an embodiment in which metal nanowires can bedeposited on a substrate;

FIGS. 2 A and B illustrate an embodiment wherein chemical mechanicaletching is used to generate metal electrodes (embedded in a dielectricsurface) on which Pd or Pd-alloy nanowires can be electrochemicallygrown;

FIG. 3 illustrates a variable-range hydrogen sensor of the presentinvention comprising metal nanowires;

FIG. 4 illustrates a variable-range hydrogen sensor of the presentinvention comprising columns of metal nanoparticles; and

FIG. 5 illustrates an embodiment wherein a variable-range hydrogensensor of the present invention is used to monitor hydrocarbon breakdownin electrical transformers.

DETAILED DESCRIPTION

The present invention provides for a method of generating metalnanowires on a surface, wherein the nanowires are grown with predefineddimensions, compositions, and orientations. Such metal nanowires aretermed “precisely-defined” herein. The present invention is alsodirected to a variable-range hydrogen sensor comprising one or moreprecisely-defined palladium-silver (Pd—Ag) nanowires of variablecomposition and possessing nanobreakjunctions which are closed at acomposition-dependent hydrogen concentration threshold. In someembodiments, an array of such nanowires of differing composition isused. In such embodiments, the nanobreakjunctions close (in sequentialfashion) as the concentration of H₂ is increased. Changes in anelectrical property or properties (e.g., resistance) of these nanowiresas a function of H₂ concentration present, permits the sensing of H₂over a range of concentration. Such a variable-range sensor can evenpermit concentration determination when electrical responses of thesensor have been calibrated against known quantities of H₂. Theprecision with which this can be done is merely dependent upon thenumber of precisely-defined metal nanowires of different Pd:Ag ratiospresent in the sensor.

In some embodiments of the present invention, the precisely-definednanowire sensors (“nanowire sensors”) described herein are made by theelectrochemical, electroless, or vapor deposition of metal intophotolithographically-generated and etched channels within a dielectricmaterial and which span two electrodes, permitting the generation ofnanowires formed in any desired orientation, length, or arrangement, andwithout the need to transfer them to an additional substrate.Dimensions, in this embodiment are limited only by the dimensionallimitations of photolithographic techniques, e.g., the wavelength of theradiation used.

Referring to FIG. 1, in some embodiments, Pd nanowires areelectrochemically deposited onto a SiO₂ dielectric substrate 102 (otherembodiments employ different dielectric materials). In Step 1001, a200-600 nm thick layer of silicon dioxide (SiO₂) 102 is plasma depositedon top of the base Si substrate 101. On top of this, Ti is thermallyevaporated and condensed onto the surface such that a 10-100 nm (e.g.,˜50 nm) Ti film 103 resides on top of the SiO₂ (note that in otherembodiments Ti is sputtered or electron-beam evaporated onto thesurface). A 1-5 μm (e.g., ˜2 μm) Shipley AZ photoresist (PR) layer 104is spin-coated onto the Ti film 103 using a spin coater operating atabout 1000-5000 RPM. A subsequent baking process is used to remove thesolvent. A photomask (essentially a glass plate with a photo-opaquedesign on it) is applied to the PR layer 104 and is exposed to UV light(e.g., generated by a mercury arc lamp or other source). The photomaskis removed and the substrate is placed in a developer which removes theUV-exposed regions or the PR layer 104, thus creating a pattern in thePR layer 104. In Step 1002, the Ti layer 103 is etched using a reactiveion etching (RIE) plasma using a fluorine- or chlorine-containing gas(or other suitable etching process). This generates nanoscale “walls”(nanowalls) of Ti 105 which can then be electrically-biased such that Pdis electrochemically deposited (from a solution) along the Ti walls 105as Pd nanowires 106 having diameters generally in the range of about 100nm to about 700 nm, and perhaps smaller. Methods for electrochemicallydepositing Pd from solution are well known in the art (F. Favier, E. C.Walter, M. P. Zach, T. Benter, R. M. Penner “Hydrogen Sensors andSwitches from Electrodeposited Palladium Mesowire Arrays,” Science, 293,p. 2227-2231, 2001). In Step 1003, the patterned photoresist 104 and theTi walls 105 are removed to reveal the isolated nanowires 106.Variations on this embodiment include substituting carbon (C), tungsten(W), alloys of titanium and tungsten (TiW), and aluminum (Al) for theTi, and electrochemically depositing nanowires of Pd-alloys like Pd—Ag.In this manner, nanowires of a variety of Pd:Ag ratios can be made.

In another embodiment, shown in FIG. 2, small rectangular structures 203of height h are formed from a dielectric material, e.g., SiO₂. Thesestructures are placed on a surface 201, covered with a thin (˜10 nmthick) metal layer 202 (e.g., Ni), then planarized with additionaldielectric 204 to height H, as shown in FIG. 2A. Chemical-mechanicalpolishing (CMP) is then employed to etch away the dielectric material(from height H to below height h) and exposes the metal electrodes 205on which the Pd or Pd-alloy nanowires 206 are grown, as shown in FIG.2B. Variations on this embodiment include different dielectricmaterials, different metal thin films, size and shape of the dielectricstructures, and where and how they are placed on the surface.

In another embodiment, polymethylmethacrylate (PMMA), or other suitableelectron-beam resist material, is deposited on a conductive metal whichhas itself been deposited on a SiO₂ surface (or the surface of anydielectric material). Electron-beam (e-beam) lithography is then used togenerate lines in the PMMA which can be as small as 20 nm in width. Theconductive metal is then etched (via a RIE process) to replicate thePMMA pattern in the metal. The PMMA is removed and Pd or a Pd-alloy iselectrodeposited onto the surface. The conductive metal can beoptionally removed to maximize the performance of the hydrogen sensor.

One conductive “metal” ideally suited to the Application described inthe preceding embodiment is carbon. In some embodiments, deposition ofthe PMMA onto a carbon-coated dielectric surface, lithographicallypatterning the PMMA with an electron beam, reactive ion etching of theexposed carbon, and removal of PMMA yields carbon nanoelectrodes alongwhich Pd and Pd-alloy nanowires are grown. The carbon is then removedvia RIE in either a hydrogen, oxygen, or air plasma. The carbon leavesas a volatile reaction product like methane (CH₄), carbon monoxide (CO),or carbon dioxide (CO₂)—depending on which reactive ion etch is used.

In another embodiment, PMMA is deposited directly onto an SiO₂ surface(or the surface of any dielectric material). Electron-beam lithographyis then used to generate channels in the PMMA which can be as small as20 nm in diameter. Pd or a Pd-alloy is then electrolessly-deposited ontothe surface. Finally, the PMMA is removed with a suitable solvent toleave free-standing Pd or Pd-alloy nanowires on the surface.

In other embodiments of the present invention, carbon nanotubes (CNTs)are plated with a thin film of Pd or Pd-alloy using either anelectrochemical or electroless plating process. Such thin films possessthe same nanobreakjunctions that the other nanowires described herein doalong the length of the CNTs. The underlying carbon (i.e., the carbonnanotubes) in these coated nanotubes is then removed via reactive ionetching, as described in the preceding paragraph, to yield Pd orPd-alloy nanowires. In some of the embodiments utilizing carbonnanotubes, the CNT is grown in situ between two electrodes using anestablished vapor growth mechanism. Such a process, leads to theformation of some of the smallest Pd and Pd-alloy nanowire hydrogensensors (CNTs can have diameters as small as 0.5 nm, but CNTs grown froma supported catalyst structure are usually larger). In otherembodiments, the CNTs are produced external to the sensor device, thenthey are coated with Pd or a Pd-alloy. Such coated CNTs are thendispersed on a surface or in lithographically-generated channelsbridging two electrodes on a surface. In these latter embodimentsutilizing CNTs, the nanowire sensor is actually composed of a number ofsmaller nanowires.

Other embodiments of the present invention involve coating nanoparticles(having diameters as small as 1 nm) with a Pd and Pd-alloy. Silicon,silica, diamond, alumina, titania, or any other nanoparticle material iselectrolessly plated with a Pd or Pd-alloy. These coated nanoparticlesare then applied to a surface and made to bridge two electrodes usingelectrophoresis, spray methods, or pastes. Nanobreakjunctions exist inthe coated surfaces and in the gaps (nanogaps) between adjacentparticles. Variations on these embodiments include depositing suchcoated nanoparticles within lithographically-patterned channels on thesurface of a dielectric material, and generating nanoparticles of Pd andPd-alloys electrochemically on a surface.

In all embodiments involving nanoparticles, the nanoparticles aredeposited or applied to a surface in such a way so as to assure thatthere is significant contact between nanoparticles. This ensures thatelectrical contact is made throughout the entire nanoparticle network.As in the electrochemically-generated Pd and Pd-alloy wires, it is stillthe nanobreakjunctions on the surface of the coated particles whichserve in the sensing mechanism. Here, the nanowires are simply createdwith numerous coated nanoparticles.

Other embodiments include depositing Pd and Pd-alloys using thermal- orelectron-beam evaporation. Furthermore, nanowires of any platable metalor combination of platable metals can be made using eitherelectrochemical or vapor deposition techniques. Platable metals include,but are not limited to, Ag, Au, Cu, Co, Fe, Ni, Pd, Pt, Cr, Zn, Sn, Ti,and combinations thereof. As will be appreciated by those of skill inthe art, countless other variations exist utilizing maskless or laserphotolithographic techniques and combinations of all the previouslymentioned techniques.

The nanowires of the present invention can also be patterned on asurface to have specific dimensions and orientations. This is importantbecause future commercialization of this technology will likely dependon the ability to control the size, shape, and orientation of thenanowires within a manufacturing context.

The above-described processes of making metal nanowires provide numerousadvantages over the existing prior art, particularly for fabricatinghydrogen sensors. The number, length, diameter, and orientation ofnanowires in the device can all be controlled by lithography. Theelectroplating (deposition) process can be more easily controlledbecause one knows exactly the length and number of wires and thereforecan control the size of the nanowires more easily. All the nanowires canbe aligned in parallel (if desired), whereas the prior art does not haveany control over this on a graphite surface. Much of the technology ofthe present invention is built on Si which is easily cleaved/sawed intoindividual devices, rather than being on glass which is difficult towork with. Furthermore, no glue (cyanoacrylate) is required in theprocesses of the present invention. Such glue has limited use over atemperature range, and typically has a thermal coefficient of expansionwhich is may further limit its use over a range of temperatures.

In general, the present invention also comprises any method that allowsfor the creation and controlled placement of Pd and Pd-alloy (e.g.,Pd—Ag) nanowires for use as hydrogen sensors. The invention alsoprovides for variable-range hydrogen sensors in that it provides amethod for preparing nanowires which are sensitive to hydrogen over arange of concentrations at a given temperature and to a given H₂concentration over a range of temperatures. This is accomplished byalloying the Pd with Ag and forming nanowires of this Pd—Ag alloy asdescribed above. Like the Pd nanowires, such Pd—Ag nanowires possessnanobreakjunctions which are responsive to H₂ concentration. Alloying Pdwith Ag permits one to modulate the α-β transition and enables thesensor to respond (by closing the nanobreakjunctions) to a much widerrange of temperatures and H₂ concentrations. Methods ofelectrochemically (J. N. Keuler, L. Lorenzen, R. D. Sanderson, V.Prozesky, W. J. Przybylowicz “Characterization of electroless platedpalladium-silver alloy membranes,” Thin Solid Films, 347, p. 91-98,1999) and evaporatively (V. Jayaraman, Y. S. Lin “Synthesis and hydrogenpermeation properties of ultrathin palladium-silver alloy membranes,” J.Membrane Sci., 104, p. 251-262, 1995) alloying Ag with Pd arewell-established. The invention also provides a method of making sensorscomprising of an array of several (2 or more) metal nanowires, eachpossessing a different Pd/Ag ratio, where the amount of Ag can vary fromabout 0% to about 26%. Not intending to be bound by theory, otherplatable metals and combinations of platable metals may also be used tomake nanowire sensors for hydrogen and, perhaps, other gases. The keyhere is that such nanowires comprise nanobreakjunctions which close atsome threshold hydrogen concentration.

FIG. 3 illustrates a variable-range hydrogen sensor of the presentinvention comprising metal nanowires. Referring to FIG. 3, metal alloynanowires 301 of variable composition and comprising nanobreakjunctions302 are present on a dielectric surface 303. An electric circuitcomprising the nanowires 301 is formed with electrical contacts of metalfilm 304 and a power supply 305. The sensor functions by monitoring someelectrical property of the nanowires with a monitoring device 306 (e.g.,an amp meter) for changes as the nanowires are exposed to hydrogen.

FIG. 4 illustrates a variable-range hydrogen sensor of the presentinvention comprising columns of metal nanoparticles. Referring to FIG.4, columns of metal alloy nanoparticles 401 of variable composition andcomprising nanobreakjunctions and nanogaps 402 are present on adielectric surface 403. An electric circuit comprising the columns ofmetal alloy nanoparticles 401 is formed with electrical contacts ofmetal film 404 and a power supply 405. The sensor functions bymonitoring some electrical property of the nanowires with a monitoringdevice 406 (e.g., an amp meter) for changes as the columns ofnanoparticles are exposed to hydrogen.

An advantage to alloying Pd with Ag in the nanowires is that it permitsthe formation of hydrogen sensors having a variable-range of detectionpoints. Using pure Pd nanowires, one is limited only to about a 2%detection capability at room temperature, and at 40-50° C., the α-βtransition point shifts to 4-5% H₂, above the point at which it isuseful in detecting explosive H₂ levels. This limitation is overcome bymaking nanowire alloys of Pd—Ag in the 0-26% weight concentration of Agto Pd. As mentioned above, such nanowire alloys will permit hydrogendetection over a wide range of temperatures and hydrogen concentrations.Furthermore, using an array of different Pd—Ag nanowires (multiplenanowires, each having a different Pd to Ag ratio) allows for theformation of a variable-range hydrogen sensor which will be dramaticallymore useful in research and industrial settings.

Additionally, as an alternative to basing the sensing mechanism on asharp change in resistance, the hydrogen sensing process can also bemade to work on changes in capacitance or conductance. Essentially,monitoring any electrical property which changes in a pre-defined manneras a result of closing the nanobreakjunctions within the nanowire can beused to sense an increase in the hydrogen concentration of thesurrounding environment.

An exemplary application for such hydrogen sensors is in the monitoringof hydrocarbon breakdown (which leads to the evolution of hydrogen) intransformers. FIG. 5 illustrates a placement of such a sensor 502 in atransformer 501 for hydrogen monitoring, wherein electrical contacts 503connect the sensing element 502 (comprising nanowires or columns ofmetal nanoparticles) to a power supply 504 and an electrical propertymonitoring device 505. In some embodiments, the sensor 502 is placed inthe transformer such that it is exposed to transformer oil. When thefilled transformer operation becomes defective, hydrogen and otherdissolved gases form in the oil. The sensor 502 monitors the H₂ contentdissolved in the oil, helping to identify potential breakdowns andpermitting maintenance before such breakdown occurs. Such monitoring canserve to economize the maintenance and downtime involved in operatingsuch transformers by providing for a realtime and remote monitoringmeans.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

1. A method comprising the steps of: a) forming, directly on adielectric surface, at least one precisely-defined metal-alloy nanowirecomprising nanobreakjunctions which close when exposed to predefinedthreshold concentrations of hydrogen; b) forming a circuit comprisingsaid nanowire; and c) monitoring an electrical property within saidcircuit so as to determine when said nanobreakjunctions close.
 2. Themethod of claim 1, wherein the step of forming involves usinglithographic and electrochemical deposition means.
 3. The method ofclaim 1, wherein the dielectric surface comprises a layer of SiO₂. 4.The method of claim 1, wherein the nanowire comprises Pd and Ag.
 5. Themethod of claim 1, wherein the metal nanowires have a composition thatcan be tailored so as to effect nanobreakjunction closure at varyingconcentrations of hydrogen.
 6. The method of claim 1, wherein saidmethod provides for hydrogen detection in transformers.